Shape-retaining fiber optic cables having limited bend radius

Abstract

In accordance with this invention, fiber optic cables are provided whose shape may be formed and retained while maintaining a limited bend radius. These features are produced by incorporating a compact compliant internal cable member into the cable structure. The compliant internal member consists not only of the fiber optic cable, but also of ductile and non-ductile elements. The ductile element is advantageously a tube or a wire which readily deforms to retain a given shape, and may be reshaped if desired. The non-ductile element, which resists sharp bending of the cable during shaping, comprises a substantially non-ductile elongated element disposed within the cable and configured to oppose excessively sharp bending along its length. Proper selection of the cross-sections and materials used in these elongated members produces a proper balance between shape retention and bending radius.

1. An elongated fiber optic conduit structure which can be multiply shaped to follow a desired path while maintaining a minimum bend radius, comprising:
 an elongated jacket for confining at least two extended members, the jacket having a longitudinally pliant body with limited radial expandability; 
 a fiber optic conduit disposed within and along the jacket, the conduit having a low structural rigidity but required to have more than at least a minimum bending radius; 
 a first elongated structural member disposed within and extending along the jacket, said member being of ductile material which yields under a predetermined force; 
 a second elongated structural member disposed within and along the jacket, the second adjacent the first structural member, the second member being non-ductile and producing a force when subjected to the minimum bend radius that is nominally equal to the predetermined force which can be sustained by the first structural member without yielding, and 
 the first and second structural members being juxtaposed within the jacket to provide a combinatorial response to curvature such that the fiber optic conduit system opposes shaping to less than the minimum bend radius. 
 
   
   
     2. An elongated fiber optic conduit structure in accordance with  claim 1  above, wherein the structure can be subsequently re-shaped to follow and retain an alternate path while maintaining a minimum bend radius. 
   
   
     3. A fiber optic conduit structure as set forth in  claim 1  above, wherein the first structural element is hollow and the second structural element and the said conduit are disposed therewithin. 
   
   
     4. A fiber optic conduit structure as set forth in  claim 1  above, wherein the first structural element is hollow and within said jacket and the conduit is disposed within the first structural element and the second structural element is within the jacket external to the first structural element. 
   
   
     5. A fiber optic conduit structure as set forth in  claim 1  above, wherein the conduit structure comprises paired structures with longitudinally adjoining jackets. 
   
   
     6. A fiber optic conduit structure as set forth in  claim 1  above, wherein the first structural element defines the jacket and includes a central bore encompassing the second structural element and the conduit. 
   
   
     7. A fiber optic conduit structure as set forth in  claim 1  above, wherein the jacket has bend-limited compliance, and wherein the second structural member and conduit are within the jacket, and is in longitudinal physical engagement with the jacket. 
   
   
     8. A structure in accordance with  claim 1 , wherein the fiber optic conduit is comprised of one or more optical fibers selected from the class of fibers including single mode, multi-mode, dispersion compensating, bend-insensitive, high numerical aperture, photonic crystal and plastic optical fibers. 
   
   
     9. A structure in accordance with  claim 8 , wherein the optical fibers are fabricated of fused silica with an outer diameter of 125 to 250 microns with a protective coating of 250 to 900 microns. 
   
   
     10. A structure in accordance with  claim 1 , wherein the first elongated structural member is soft annealed copper, cylindrical in cross section with an outer diameter of 1.5 to 2.0 mm, an inner diameter of 1 to 1.5 mm, and wherein the second elongated structural member is spring steel, circular in cross section with a diameter of 0.5 to 1 mm. 
   
   
     11. A fiber optic structure shapeable to a selected longitudinal configuration that is limited to a minimum bending radius, comprising, in combination,
 first and second elongated members in proximate lengthwise relation, a first of the members having an elastic bending response when bent to a radius of curvature lateral to its longitudinal extent, the first member exerting a lateral force inversely related to the imposed bending radius, the second member, being pliant laterally with an inelastic response of predetermined magnitude to bending force, said predetermined force response being greater than the lateral force of the first member until the structure is shaped to a radius of curvature along its longitudinal extent equal to the minimum bending radius, the lateral force of the second member being in opposition to the lateral force of the first member, 
 a fiber optic conduit transversely adjacent the first and second members, and bendable therewith and 
 an element enclosing the first and second members and the conduit to maintain the physical proximity therebetween, such that there is lateral force interaction between the first and second members when bent, wherein the first member is non-ductile with substantially circular cross sections of 0.5 to 1 mm in diameter, and the second member is ductile with substantially cylindrical cross sections with 1 to 1.5 mm inner diameter and 1.5 to 2.0mm outer diameter. 
 
   
   
     12. A cable in accordance with  claim 11  wherein the limited bend radius is about 37 mm or more. 
   
   
     13. A flexible, fiber optic interconnection cable including one or more optical fibers and separate, longitudinally adjacent first and second laterally bendable elements and an element maintaining said elements in transverse proximity and bendable together along their lengths, wherein, under an applied bending force, the first bendable element exerts a responsive spring force in opposition and the second bendable element physically deforms under the applied bending force to the new shape, the second bendable element being of a ductile material and stiffness to provide a holding force opposing the spring force produced by the first bendable element to substantially maintain any new shape having greater than a predetermined minimum bend radius, wherein the interconnection cable further includes ends which are terminated in polished fiber optic connectors including a rigid connector body, having a rigid, longitudinally extended support incorporated therein, the first bendable element being inserted therein to allow optical fibers to pass longitudinally therein, the support preventing excessive bending at the insertion point of the cable into the connector body. 
   
   
     14. A structure in accordance with  claim 11 , wherein the first member is substantially comprised of a first material and the second member is substantially comprised of a dissimilar, second material, the first material characterized by a yield strength greater than that of the second material, wherein the first material is high carbon steel or a carbon fiber composite, and the second material is soft annealed copper, tin, brass, aluminum or low carbon steel. 
   
   
     15. A flexible, fiber optic interconnection cable as set forth in  claim 13 , wherein the responses of the first and second elements are selected to maintain a bend radius greater than 37 mm under an applied bending moment of less than 0.5 kg-cm.